Beijing, China (CNN) -- The death toll from an explosion at a northeastern China coal mine rose to 104 Monday morning, according to state-run media.

Another four workers were still trapped underground in the mine shaft as of 7:20 a.m. (6:20 p.m. ET Sunday), the Xinhua news agency said. A buildup of toxic gases was hampering rescue efforts, according to CCTV.

Sixty miners remained hospitalized Monday for injuries ranging from carbon monoxide poisoning to burns, CCTV reported. Of those, six had sustained severe burns that left them in critical condition.

About 530 miners were working in the mine when the blast occurred at about 2:30 a.m. Saturday (1:30 p.m. ET Friday). The Heilongjiang Longmei's Xinxing mine is operated by the Hegang company.

A preliminary investigation indicates the blast was a gas explosion, Xinhua reported.